Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false. The description adds useful behavioral context: api_key comes from signup and the response has free vs Gemini-TTS voice tiers. It does not describe response shape or pagination, but for a simple one-parameter list with strong annotations this is acceptable.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is a single compact sentence that front-loads the resource and purpose, then gives downstream usage, cost tier, and auth source. Every clause adds value, with no repetition of schema or annotation data.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a one-parameter, read-only listing tool, the description is nearly complete: it explains what is returned, where the result is consumed, and the auth requirement. The absence of an output schema means a bit more detail about the exact response fields would help, but the phrase 'voice_id' and '一覧' convey enough for correct invocation.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, and the api_key parameter already has type and format ('API キー (sk_live_…)'). The description's mention that the key is received at signup adds marginal context, but the schema carries the main semantic burden, so baseline 3 is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific verb and resource: it returns a list of usable voices ('使える声 (voice_id) の一覧'). It also explicitly maps the output to the voice_id parameter used in create_session and create_avatar, which distinguishes it from sibling list_* tools.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives clear context on when to use it: when you need a voice_id for create_session or create_avatar. It also adds a cost/free distinction between the Japanese standard voices and Gemini-TTS voices, helping the agent choose appropriately. It does not explicitly name an alternative or state when not to use it, so it misses the top bar.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.